Dip-formed articles such as rubber gloves and finger cots have been widely used in various fields including a medical field (such as the prevention of hospital infection or infection with SARS), a food processing field (such as 0-157 problems), and an electronic part production field in association with a growing interest in safety and sanitation. A dip forming method is one method of producing each of the rubber gloves, the finger cots, and the like. Known examples of the dip forming method include an anode coagulant dipping method involving previously dipping a mold made from, for example, wood, glass, ceramic, metal, or plastic into a coagulant liquid and dipping the resultant into a natural rubber latex composition or a synthetic rubber latex composition and a Teague coagulant dipping method involving dipping a mold into a latex composition and dipping the resultant into a coagulant liquid, and molded products obtained by these dip forming methods are dip-formed articles.
A natural rubber latex is a representative latex for dip forming. A natural rubber latex product has good physical and chemical properties, but the user may suffer from an allergic reaction in association with the elution of a natural protein in the product, so the number of products each produced by using a protein-free synthetic rubber latex tends to increase.
It has been pointed out that an acrylonitrile-butadiene rubber (NBR rubber), which is a representative example of the synthetic rubber latex, may generate a harmful substance such as hydrogen cyanide originating from acrylonitrile in an exhaust gas, so a new latex raw material such as a styrene-butadiene rubber (SBR) (JP-A-2001-192918) or a carboxyl group-containing ionomer-based elastomer has been attracting attention.
High levels of physical properties are desired for article. A crosslinked structure must be introduced between the molecules of a polymer of which the dip-formed article is composed in order that the dip-formed article may exert high levels of physical properties.
In the case of a natural rubber, sulfur and a vulcanization accelerator such as zinc oxide are added to form the covalent bond of sulfur between the double bonds of the molecules of the natural rubber. In the case of the natural rubber, the so-called sulfur vulcanization is considered to form a crosslinked structure even in a natural rubber particle, so the resultant product exerts excellent physical properties.
The same sulfur vulcanization method as that in the case of the natural rubber is generally adopted also in the case of a diene-based carboxylated synthetic rubber latex. However, the role of each chemical to be added is considerably different from that in the case of the vulcanization of a natural rubber latex. That is, when zinc oxide contacts with water, a hydroxyl group is produced on the surface of zinc oxide, and the hydroxyl group reacts with a carboxyl group of a particle of the diene-based carboxylated synthetic rubber latex (P. H. Starmer, Plastics and Rubber Processing and Applications, 9 (1988), 209-214) to form a pendant half salt, and, furthermore, cluster ion crosslinkage may be formed after a heat drying process for the pendant half salt. The physical properties of the surface of zinc oxide to be measured, such as a tensile strength, an elongation, and a hardness are determined by the zinc crosslinkage, which is a major difference from the case of the natural rubber latex where the physical properties of a product are determined by sulfur crosslinkage.
The term “cluster ion crosslinkage” as used herein refers to a state where carboxyl groups form a cluster, and a divalent cation of zinc is neutralized by the whole carboxyl groups forming the cluster. The structure of the cluster ion crosslinkage has the following characteristic: when rubber is elongated, crosslinkage is misaligned, and, when a stress is applied to the rubber, the rubber undergoes stress relaxation (creep) within a short time period, and, if used for a long time period, its permanent distortion enlarges, with the result that the rubber elongates (N. D. Zakharov, Rubber Chem. and Tech, Rubber Division Acs. Akron, US. Vol 36, no 3, 568-574).
On the other hand, sulfur, which crosslinks double bonds originating from butadiene with a covalent bond, has small influences on the physical properties of a rubber product to be measured, such as a tensile strength, an elongation, and a hardness. However, sulfur dominates the important properties of the rubber product, such as the durability, creep resistance, water resistance, and solvent resistance of the rubber product, and the fact is the reason why a sulfur vulcanization method is frequently adopted also in carboxylated synthetic rubber latices.
As described above, sulfur vulcanization plays an important role also in a diene-based carboxylated synthetic rubber latex. On the other hand, in the electronic part production field, the frequency at which the sulfur vulcanization is employed tends to reduce because sulfur oxidizes a metal when sulfur contacts with the metal.
In addition, the development of a dip-formed article using no vulcanization accelerator has been demanded because there has been a tendency for the number of cases where contact dermatitis based on a delayed allergy against a vulcanization accelerator in a dip-formed article such as a glove occurs to increase in recent years.
Further, in a food field, there has been a tendency to step up controls on the eluted amount of zinc as a heavy metal to elute from a rubber glove.
By the way, the inventors of the present invention have proposed a method involving the use of, for example, an aluminate as a crosslinking method in which neither sulfur nor a vulcanization accelerator is used (JP 3635060 B). However, the method has the following drawback: a rubber product becomes hard because aluminum functions as a trivalent cation.
In addition, JP-A-2003-165814 proposes a dip-forming composition substantially free of a sulfur-containing vulcanizer, a vulcanization accelerator, and zinc oxide. However, investigation conducted by the inventors of the present invention has shown that a dip-formed article using the composition involves the problems in that the product is poor in creep resistance, water resistance, and solvent resistance, and has strong cohesiveness.
It should be noted that WO 2004/071469 discloses a hydrogel patch composition. The hydrogel patch composition contains a water-soluble polymer gel and a crosslinking agent, and the crosslinking agent contains dihydroxyaluminum acetate. However, dihydroxyaluminum acetate is interpreted as a cationic crosslinking agent for the water-soluble polymer gel. However, neither the cationic crosslinking agent of a gelled composition nor a cationic crosslinking agent for gelling a composition can be a crosslinking agent for a carboxyl group-containing diene-based rubber latex where the stable, long-term presence of a blended liquid is most important. In addition, the document has no description concerning what action mechanism the crosslinking agent has on the water-soluble polymer gel.
In addition, JP-A-2005-97217 discloses a gel sheet for bleaching containing, as a component, an ion crosslinked article of an anionic water-soluble polymer compound, which is obtained by polymerizing acrylic acid or a derivative of acrylic acid, with a polyvalent cationic compound, and describes dihydroxyaluminum aminoacetate as the polyvalent cationic compound. However, the crosslinking agent is also a gelling agent for the anionic water-soluble polymer compound, and is gelled by being left at rest for 24 hours. Therefore, the document does not disclose a crosslinking agent for a carboxyl group-containing diene-based rubber latex according to the present invention any more than WO 2004/071469.
Further, JP-A-2005-15514 discloses a composition for a water-dispersed type rust preventive coating containing, as essential ingredients, an ionomer resin, specifically, an ethylene-unsaturated carboxylic acid copolymer and a water-soluble titanium compound having reactivity with a carboxyl group, and describes dihydroxytitanium lactate as the water-soluble titanium compound. However, the ionomer resin is a special resin that can be neutralized with a divalent or trivalent metal ion, so the document does not disclose the crosslinking agent for a carboxyl group-containing diene-based rubber latex according to the present invention.